In 5-10% of those with psoriasis, arthritis also appears. In most cases the psoriasis will precede the arthritis, sometimes by many years. When arthritis symptoms occur with psoriasis, it is called psoriatic arthritis (PsA). In these cases, the joints at the end of the fingers are most commonly affected causing inflammation and pain, but other joints like the wrists, knees and ankles can also become involved. This is usually accompanied by symptoms of the fingernails and toes, ranging from small pits in the nails to nearly complete destruction and crumbling as seen in reactive arthritis or fungal infections.

About 20% of people who develop PsA will eventually have spinal involvement, which is called psoriatic spondylitis. The inflammation in the spine can lead to complete fusion - as in ankylosing spondylitis.